RAID: Faster and Cheaper with Linux Page 2

Software RAID Advantages
Linux software RAID is more versatile than most hardware RAID
controllers. Hardware controllers see each drive as a single member of
the RAID array, and handle only one type of hard disk. Most hardware
controllers are picky about the brand and size of hard disk -- you can't
just slap in any old disks you want, but must carefully choose
compatible disks. And it's not always documented what these are.

Linux RAID is a separate layer from Linux block devices, so any block
device can be a member of the array -- a particular partition, any type
of hard drive, and you can even mix and match. Endless debates rage
over which offers superior performance, hardware or software RAID. The
answer is "it depends." An old slow RAID controller won't match the
performance of a modern system with a fast CPU and fast buses. The
number of drives on a cable, the types of drives and cabling, the speed
of the data bus- all of these affect performance in addition to the
speed of the CPU and the demands placed on it.

One disadvantage is hot-swap ability is limited and not entirely
reliable.

Converting An Existing System To RAID
First of all, your power supply must be capable of powering all the
drives you want to run on the system. Adding as many drives as you want
is easy and inexpensive. If you're going to purchase new hard disks,
you might as well get SATA, because the cost is about the same as PATA.
SATA drives are faster and use less cabling, and will soon supplant
PATA drives. PCI controller cards for additional PATA and SATA disks
cost around $40, and will run two disks each. The built-in IDE channels
on mainboards can handle two disks each, but you should run only one
disk per channel. You'll get better performance and minimize the risk
of a fault taking out both hard disks.

Next, install the raidtools2 and mdadm packages. If you
want your RAID array to be bootable, you'll need RAID support built
into the kernel. Or use a loadable module and use an initrd
file, which to me is more trouble than rebuilding a kernel. Tomorrow
in Part 2 we'll cover how to do all of this. You may get a head start
by consulting the links in Resources.